1. Field of the Disclosure
This disclosure relates generally to methods and apparatus for measuring cardiac output of a living subject, and specifically in one aspect to using substantially non-invasive techniques to accurately estimate cardiac output of a living subject.
2. Description of Related Technology
Cardiac output (CO) refers to the volume of blood pumped by the heart (by a left or right ventricle) over a period of time, such as in one minute. CO may further refer to the combined sum of output from the right ventricle and the output from the left ventricle during the phase of systole of the heart, and is mathematically expressed as:CO=Stroke Volume×Heart RateHeart rate (HR) refers to the number of heartbeats per unit of time, typically measured as beats per minute (bpm). Stroke volume (SV) refers to the volume of blood pumped from one ventricle of the heart with each beat.
Heart functioning and CO are generally controlled by the demand for oxygen by the body's cells. As the oxygen demand increases, CO increases. Therefore, increases in HR, change of posture, increased sympathetic nervous system activity, and decreased parasympathetic nervous system activity, body surface area, etc. may affect a patient's CO. Other physiologic parameters and measures may further affect SV and/or HR, and therefore also affect CO. For example, the Ejection Fraction (EF), which is the fraction of blood ejected by the Left Ventricle (LV) during the contraction or ejection phase of the cardiac cycle or systole, may affect SV and may vary with ventricular disease associated with ventricular dilatation. Other diseases of the cardiovascular system cause changes to CO, such as hypertension and heart failure. In addition, during infection and sepsis CO may increase, and as a result of cardiomyopathy and heart failure, CO may decrease. CO can also be affected significantly by the phase of respiration (i.e., intra-thoracic pressure changes influence diastolic filling and therefore influence CO). CO changes are especially important and efficient techniques for monitoring CO and are therefore needed during mechanical ventilation as well.
Hence, the ability to accurately measure CO is important for providing improved diagnosis of abnormalities and management thereof, including hypertension, heart failure, etc.
There are a number of clinical methods for measurement of CO, ranging from direct intracardiac catheterisation to non-invasive measurement of the arterial pulse. However, each of these methods has significant limitations including inaccuracy, ineffectiveness, and invasiveness. Specifically, pathophysiologic studies indicate that left ventricular systolic and diastolic function, as well as central vascular function, are nonlinear processes. Therefore, computation of cardiac output using the prior art linear approximation strategies is imprecise and suboptimal. Accordingly, there exists no widely accepted standard for truly accurate measurement of CO.
Hence, what are needed are accurate, effective, and non-invasive apparatus and methods for calculating CO.